As wireless digital communication systems have continued to proliferate, manufacturers of such systems have continued to look for ways to reduce the overall costs of such systems. But a reduction in the cost of a system can sometimes result in a performance penalty, due to the lower quality of less expensive components. One component in particular that can vary significantly in cost and quality is the reference crystal used to generate the clocks for producing transmitted signals and for decoding received signals. The quality of such crystals is sometimes measured as a function of how close the actual frequency generated by the crystal is to the specified frequency. One metric used to quantify the discrepancy in frequency is “parts-per-million” or “PPM,” which is a measure of the frequency range, above or below the rated frequency, within which the crystal is guaranteed to operate. Thus, for example, a 1 MHz crystal that is rated at ±100 PPM is guaranteed to operate at no more 1,000,100 Hz, and at no less than 999,900 Hz.
But designers of wireless systems are required to design transmitters and receivers to operate within known tolerances in order for the system to operate reliably. Thus, for example, a system may require that the total combined accuracy of a transmitter and a receiver communicating with each other not exceed ±40 PPM (e.g., in order to guarantee that a phase-locked loop in the receiver can acquire and lock onto the transmitted signal). In this example, if the transmitter has a crystal oscillator with an accuracy of ±20 PPM, then the accuracy of the receivers oscillator also cannot exceed ±20 PPM. To achieve such accuracy, the receiver must either use a crystal rated at ±20 PPM or better, or must use a less accurate crystal with additional circuitry that compensates for the inaccuracy of the crystal. Both solutions add to the cost of the receiver, relative to using a simple, uncompensated oscillator circuit that utilizes a less accurate reference crystal.